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Semiconductor wafers are usually contained in slots within a cassette holder. The wafers are stacked in spaced parallel arrangement and are usually scanned with a laser scanner to determine the alignment of the wafers within the cassette and to determine the center of the wafer so that each wafer can be appropriately grasped by a tool for proper removal of the wafer from the cassette and movement to an intended station. The scanner is aligned along the plane and moved vertically from wafer to wafer to provide a scan of all of the wafers within a cassette. This scanning procedure takes a relatively long time to traverse an entire stack of wafers, typically 25 wafers in a standard cassette. In addition, the determination of the wafer center for proper positioning on a tool is conventionally accomplished by separate alignment apparatus.
It would be helpful to have a simplified system for determining whether wafers are properly aligned within a cassette and at which slot locations they are within a cassette.
In accordance with the present invention an imaging system is provided in which a camera or other image sensor is mounted at a known position in relation to an open front of a cassette to view at one time an entire stack of wafers in the cassette. The camera can be mounted on a robot or on a tool. An image of the set of wafers is captured and is image processed to provide information on the presence, position and alignment of each wafer in the cassette. The image can be processed to provide data on separation of wafers within the cassette, any cross slotting of wafers, the center point of each of the wafers and whether any wafers are missing.
The camera may be a video camera of digital or analog type, and may have a solid state image detector or image detection tube such as a vidicon tube.
The wafers can be illuminated by light which may take many different forms. The light may be ambient light or light from a flash or continuously illuminated lamp or lamps. Alternatively, the light may be polarized or may be structured to illuminate only points of interest on the wafers.
The wafers must have a known diameter and a portion of the circumferential edge of the respective wafers must be viewable by the camera to identify at least three points on the circumference from which the center of the wafers can be computed. The position of the wafers in the cassette is also computed from the edge detection data and from the known position of the cassettes in relation to the camera position.
The system can be used to determine how many wafers are in the cassette, whether any wafers are missing and whether the wafers are properly disposed in their respective slots within the cassette. It can be determined whether any wafers are cross slotted such that they are not parallel stacked as they should be for proper removal and reinsertion into the cassette by a robot or other transfer tool. Image analysis and processing is employed to determine the center of each of the wafers. By determining the center of the wafers, a robot can grip the center of the wafers without need for a separate alignment step or alignment apparatus. One image scan provides a determination of all information on the X, Y and Z position of the wafers in relation to the camera reference position. The rotational position of the wafers about their respective centers is not determined but is not needed for system function.
The imaging system can also be employed to determine the presence and position of other substrates or items of known shape and dimensions. For example, for image processing of a square, rectangle or other multilateral substrate of known size, a corner and two adjacent sides or a side and two adjacent corners can be detected and the resulting data employed to determine the position of the substrates in a storage container.